1. Technical Field
The present invention relates to an electrode structure which has low resistance and high thermal stability as an electrode wiring material for liquid crystal displays and where an occurrence of stress migrations such as hillocks and whiskers is completely suppressed, and also relates to a method of forming the electrode structure.
2. Prior Art
Pure metal, such as Cu, Al, Mo, Ta, and W, or alloy material, such as Al--Cu, Al--Cu--Si, Al--Pd, has hitherto been employed in the low- resistance electrode wiring material that is employed in semiconductor devices. On the other hand, particularly in the electrode material for liquid crystal displays which have been attracting attention as flat panel displays, there have recently been required better characteristics than before, such as large area wiring for large screens, high integrated wiring for high fineness, and array formation which is the film formation onto a glass substrate. In FIG. 1 there is shown a schematic diagram of one pixel section of the array of a liquid crystal display which has a thin-film transistor (TFT) as an active device. A display electrode 2, a gate line 3, a gate electrode 3A, a data line 4, a drain electrode 4A, a source electrode 5, and an active TFT device 6 are disposed on a single pixel opening portion 1. If the TFT is turned on by a signal on the gate line 3, an electric potential on the data line 4 will become equal to the pixel electrode 2 connected through the source electrode 5. As a consequence, the liquid crystal, enclosed in the upper portion of the pixel electrode 2 in the paper surface direction, is oriented and the pixel is caused to be in a display state. Here, the electrode wiring materials for the array of a liquid crystal display, which are the objective of the present invention, are the gate line 3, the gate electrode 3A, the data line 4, the drain electrode 4A, and the source electrode 5.
The first required characteristic of the electrode wiring material for liquid crystal displays is that the electric resistance is low. If the electric resistance is high, various problems such as signal delay and heat generation will arise when the liquid crystal display is increased in its size. Pure aluminum with low electric resistance has often been employed as the wiring material for liquid crystal displays. Pure aluminum has a better etching characteristic and is also a suitable material from the standpoint of adhesion with respect to a substrate. However, pure aluminum has the disadvantage that the melting point is low and it will easily give rise to defects, called hillocks or whiskers, by a thermal process using chemical vapor disposition (CVD) after formation of a wiring film. This thermal process is usually carried out at a temperature of 300 to 400.degree. C. If the wiring material is observed by scanning electron microscope after this process, there are cases where very small protrusions or bar-shaped crystal growth will be observed on the surface.
An example of defects such as this is shown in FIG. 2. Shown in FIG. 2 is a wiring layer 20 formed on a glass plate 17. In general, a wiring layer is composed of pure aluminum (Al) or the alloy and constituted by some crystal particles 21 through 26. Here, a portion 30 extending long in the form of a whisker from the crystal particle 22 is called a whisker, and a portion 40 bulged from the crystal particle 24 is called a hillock. If the whisker 30 or the hillock 40 (hereinafter referred to as a hillock and the like) occurs, the smoothness of the wiring material layer will be lost, and a nitride film and an oxide film, which will be formed on the wiring material layer after a subsequent process, will be formed on and along the unevenness of the underlying material. Therefore, if formation of these insulating layers is not sufficient, a problem such as an interlayer short circuit will arise between an electrode which is formed on an overlying layer and an electrode which is formed on an underlying layer. For this reason, an occurrence of the hillock and the like will become an extremely large problem in the process of fabricating liquid crystal displays. The mechanism of occurrence of the hillock and the like has not yet been fully proved, but it is considered that when compressive stress is applied to a thin film due to a difference in linear expansion coefficient between a heated film and a substrate, Al atoms will move along a grain boundary by help of this compressive stress (drive force), to cause the hillock and the like being produced.
If high-melting point metal, such as Cr, Ti, Ta, and MoTa, is used in wiring material, an occurrence of the hillock and the like can be prevented because an occurrence of atom diffusion along a grain boundary is difficult. However, these high-melting point metals are wiring materials which do not meet the tendency that a liquid crystal display is increased in size, because many of the specific resistances are high like more than 50 .mu..OMEGA. cm (about 3 .mu..OMEGA. cm for aluminum).
Hence, the development of Al-based alloys have been attempted for the electrode material. Formerly Al--Cu and Al--Cu--Si were reported and recently Al--Ta and Al--Zr were reported. However, if an electrode is formed by a single layer, an occurrence rate of the hillock and the like at a high temperature of more than 300.degree. C. remains unsatisfactory at both surfaces of the electric resistance. To enhance resistance to thermal stress, the rate of the metal added to aluminum needs to be increased, but, on the other hand, the low-resistance characteristic will be lost and target fabrication will become difficult. For reasons such as this, a single layer of Al alloy is difficult to use, so an Al alloy with the reduced content of additional metal is used with a stacked or sandwiched structure with another metal. In this case a target for film formation of the other metal and a film formation processing room are needed.